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# Discrete Random Variables

How many heartbeats do you have each minute? How many points will your favorite team score in their game tonight? These any many other real-world values can be modeled by discrete random variables.

Today, Joe's class took a math quiz consisting of two problems. For a randomly selected student, let \( X \) be the points earned on the first question and \( Y \) be the points earned on the second question. The joint probability distribution of \( X \) and \( Y \) is given in the following table:

\[ \begin{matrix} & X=0 & X=5 & X=10 & X=15 \\ Y=0 & 0.05 & 0.05 & 0 & 0.07 \\ Y=5 & 0 & 0 & 0.18 & 0.07 \\ Y=10 & 0 & 0.11 & 0.36 & 0.11 \end{matrix}\]

What is the expected points that a randomly selected student got?

Today, Joe's class took a math quiz consisting of two problems. For a randomly selected student, let \( X \) be the points earned on the first question and \( Y \) be the points earned on the second question. The joint probability distribution of \( X \) and \( Y \) is given in the following table:

\[ \begin{matrix} & X=0 & X=5 & X=10 & X=15 \\ Y=0 & 0.07 & 0.07 & 0 & 0.07 \\ Y=5 & 0 & 0 & 0.18 & 0.07 \\ Y=10 & 0 & 0.11 & 0.32 & 0.11 \end{matrix}\]

The students who have earned a total of less than \(15\) points will be taking a makeup test. What is the probability that a randomly chosen student will be taking a makeup test?

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